This investigation is concerned with the preparation of structurally different 13C labeled barbituric acid derivatives and the measurement of 13C-nmr chemical shifts, 13C spin lattice relaxation times (T1), and nuclear Overhauser effect (NOE) parameters of these compounds in various solvent media. The goal of these studies is to gain information about the conformational differences of the various barbiturates and thus to learn more about the molecular basis of the biological effects of these drugs. Thus far, we have synthesized and measured the 13C T1 and NOE values for carbonyl carbons 4(6) and 2 of pentobarbital and butobarbital in methanol. The results to date indicate that the protons on the 5-ethyl group have no effect on the relaxation mechanism of any of the carbonyl carbons. Furthermore, results indicate that the protons on the 1'-methylbutyl and butyl groups of pentobarbital and butobarbital respectively do play an importat role in the relaxation mechanism of carbonyl carbons 4(6) and have no effect on carbonyl carbon 2. Presently the synthesis of pentobarbital and butobarbital containing deuterium labels at specific positions on the 1'-methylbutyl and butyl side chains respectively are in progress. The 13C T1 and NOE values for the carbonyl carbons 4(6) of these compounds will provide important conformational information.